With rapid advancements in semiconductor manufacturing techniques, new classes of miniaturized devices and systems called Microelectromechanical Systems (MEMS) have become prominent. Various MEMS devices and systems utilize tiny (micron level) mechanical and electronic components together in many applications of sensors and actuators. Microfabrication technologies have been well developed for decades due to the growth of integrated circuit manufacturing technologies. One of the first MEMS devices was developed in the mid-1960s using the existing micro fabrication technologies. During the 1980s, many MEMS devices were successfully commercialized. The miniaturized systems have started replacing various conventional sensors in many applications due to the cost effectiveness, small size, low power consumption, high reliability and compatibility with integrated circuits. Any combination of microelectronics, micromechanics, micro-optics, micro-fluidics, and micro-magnetics can be realized on a single substrate using the MEMS technologies.
The most common substrate among many available materials (Table 1) for micromachining is silicon due to its excellent mechanical properties, standardized processing and ease of integrating it with microelectronics. The three major steps for the micromachining of microelectromechanical systems are deposition, lithography and etching.
TABLE 1Common Materials used in MicromachiningMaterialsUsageCharacteristicsPolymideStructureSoft and flexible, opticalguideTungstenStructureImmune to HF attacksNi, Cu, AuStructurePlated thick structuresQuartzActuationPiezoelectricZnOActuationPiezoelectricityPZTActuationLarge piezoelectricityTiNiActuationShape memory alloyGaAsOpticsLaser, LCD, detector
There are popular MEMS devices such as pressure sensors, accelerometers, mass sensors, RF switches, optical MEMS devices and microfluidic devices. A few common applications of the MEMS devices include biomedical sensors, drug delivery systems, and various automotive sensors.
In particular, RF-MEMS devices can offer superior performance at high frequencies comparing to many traditional RF devices. For example, MEMS-based RF switches and variable capacitors require lower actuation voltages than the traditional semiconductor based devices. Additionally, many former attempts on RF MEMS phase shifters offering tremendous advantages over GaAs based semiconductor phase shifters have been reported. Furthermore, MEMS devices can even be implanted in human body.
Mass sensors are one of the popular MEMS sensors that are commonly used for biological and chemical sensing. Micromachined mass sensors can recognize a change in mass via a change in their oscillating frequencies. A mass sensor's sensing area is generally treated with a binding layer that selectively binds the target chemical or biological analyte as shown in FIG. 1.
Micromachined cantilever is the most popular mass sensor type. It utilizes a single clamped mechanical beam. Cantilever mass sensors are classified as “Static” or “Dynamic” by their mode of operation. In the static mode, there is a mechanical movement or deformation of the beam as the surface stress varies, which is induced by the adsorption of atoms on atomically pure surfaces. Chemically induced stress has also been extensively studied. All the stresses can cause a deflection motion in the cantilever beam structure.
In the resonance mode, the MEMS based cantilever can be regarded as a weakly damped oscillator in the presence of gases or under vacuum. The resonance can be observed by transducing the mechanical resonance into an excitation in the electric field, acoustic field or electromagnetic field. The resonant frequency is affected by mass loading, mechanical damping or a spring constant. By observing the change of the resonant frequency, a variation of the mass can be detected.
Cantilever mass sensor systems use the change in cantilever parameters like cantilever tip position, radius of curvature, intrinsic stress or resonance frequency to detect mass. The mass sensors employ various readout schemes, including optical, piezoresistive, and piezoelectric methods. There are inherent advantages and disadvantages for each readout scheme and an optimum readout scheme has to be chosen depending on the application.
Micromachined mass sensors based on acoustic resonators have many advantages, including low cost of manufacturing and high sensitivity. Acoustic mass sensors are based on the fact that the resonant frequency changes with a change of the mass on the resonator surface. There are two popular types of acoustic mass sensors which are in common use: surface acoustic wave devices and bulk acoustic wave devices.
Surface acoustic wave devices (SAW) utilize surface waves that have particle displacement in directions both perpendicular and parallel to the wave's propagation and have been used as a highly sensitive mass detector. SAW devices use two interdigital transducers (IDTs) with a sensing area in between. One of the IDT is called the input IDT and generates acoustic waves. The other IDT is called the output IDT and converts the acoustic waves into the electric signals. The surface acoustic waves produced by an RF signal travel along the delay path caused by the added mass.
The fabrication of SAW sensors is simple and the typical Quality factor (Q) value is in the range of 10,000. Generally, SAW devices are coated with a thin film (FIG. 2) that can selectively interact with a chemical or biological analyte. The added mass that is absorbed by the sensing film causes a change in the velocity or an attenuation of the amplitude of the SAW. This change in parameters of the SAW provides the sensing mechanism.
Bulk Acoustic Wave (BAW) devices utilize longitudinal or shear acoustic waves which propagate through the bulk of the substrate unlike a SAW. Quartz Crystal Microbalance (QCM) and Film Bulk Acoustic Resonator (FBAR) are the two popular BAW devices.
A QCM consists of a thin quartz substrate and two electrodes plated on both side. When an alternating electric field is applied across the quartz crystal, acoustic waves are produced in the crystal. Resonance condition occurs when the thickness of the quartz disk is a multiple of one half wavelength of the acoustic wave. In the QCM, the acoustic wave propagation is in a direction perpendicular to the crystal surface. The resonant frequency depends on the thickness, shape and mass of the quartz. Thus any change of mass results in a change in resonance frequency. It can be safely assumed that the change in frequency is directly proportional to the amount of mass deposited on the QCM.Δf∝KΔm
However, the higher range of the quartz's resonance frequency is technically limited by the thickness of the quartz disk since it is not easy to reduce the thickness of the quartz. On the other hands, Film Bulk Acoustic Resonator (FBAR) is also a BAW sensor but uses a very thin piezoelectric film. As the piezoelectric layer thickness can be reduced down to a few hundreds of nanometers, a high resonance frequency can be attained in the range of GHz.
An FBAR device consists of a thin piezoelectric film that is sandwiched between two electrodes. When an alternating electric field is applied to the piezoelectric layer, acoustic waves are produced. The acoustic waves form a standing wave pattern if the frequency of the applied electric filed matches the fundamental resonant frequency of the device. The fundamental resonant frequency is inversely proportional to the thickness of the piezoelectric film.
FBAR has many advantages over the QCM. Since the FBARs have a thin film piezoelectric layer as the active layer, very high resonance frequencies can be obtained. The fabrication of an FBAR is compatible with standard integrated circuit (IC) process. The lateral dimensions can be equal to the thickness dimensions resulting in very small FBAR sensors. The sensitivity of the FBAR mass sensors was shown to be about 50 times better than that of typical QCM sensors.